Secondary air systems in gas turbine engines use pressurized air bled from the compressor for many purposes, including cooling and pressuring seals to reduce the risk of leaks. However, the pressurized air bled from any location within the compressor has a pressure which typically varies throughout the range of operation of the engine. For example, the pressure of the bleed air of the compressor increases when the engine is operated at a full power level in contrast to an idling condition. Accordingly, it is known to have a secondary air system using bleed air from a first location in the compressor at high power, and from a second location downstream of the first location in the compressor at low power, for example such as to ensure sufficient pressure and airflow within the system throughout the range of engine power. Such systems thus necessitate a valve to switch between the two pressure sources, and control logic to control the valve to connect to the correct source.